Molded conductive plastic resistor and method of making same



March 1% D. P. NORMAN ETAL 3,239,733

MOLDED CONDUCTIVE PLASTIC RESISTOR AND METHOD OF MAKING SAME Original Filed May 20, 1959 United States Patent 3,239,788 MOLDED CONDUCTIVE PLASTKC RESISTOR AND METHOD OF MAKING SAME Daniel P. Norman, deceased, late of Ipswich, Mass., by William W. A. Johnson, executor, Ipswich, Mass, and James Dorsey, Winchester, Mass, assignors to Ace Electronics Associates, Inc., a corporation of Massachusetts Continuation of application Ser. No. 814,503, May 20, 1959. This application Apr. 23, 1963, Ser. No. 275,800

Claims. (Cl. 338162) The present invention relates generally to molded conductive plastic resistors, and particularly to such resistors for use in potentiometers or rheostats, and is a continuation of application Ser. No. 814,503, filed May 20, 1959, in the name of the present inventor.

A number of compositions for and methods of fabricating conductive plastic resistors are taught in the prior art. These resistors are generally formed by combining a non-conductive synthetic resin with a conductive powder, molding the mixture into the desired shape and size, and curing the same to provide the desired resistance element. It is known to fabricate such resistors with varying specific or volume resistivities by varying the ratio of conductive powder to non-conductive resin. By using relatively small percentages of conductive powder, excellent unit or fixed value resistors can be readily fabricated possessing the desirable physical characteristics of the resin, such as strength, resistance to shock and wear, flexibility, and ability to withstand reasonably high temperatures.

With the development of these unit or fixed value molded plastic resistors, the industry has sought to develop corresponding types of variable resistors, such as in potentiometers or rheostats. For many applications it has been found that the mere conversion of suitable unit type resistors to rheostats is entirely unsatisfactory because of the high noise characteristic of such devices resulting from the traverse of the resistor by the rheostat wiper contact. It is believed that this high noise characteristic results at least in part from discontinuities between the conductive particles on the surface of the resistor, resulting in relatively large fluctuations in the tapped voltage value as the wiper contact traverses the unit.

The noise problem may be overcome by using a very high percentage of conductive powder relative to the resin, but the resulting device then takes on the physical characteristics of the conducting material rather than the plastic, and the value of the device as a conductive plastic resistor is in large measure defeated. By the present invention however, it has been found that conductive plastic resistors may be fabricated having exceedingly low noise characteristics, and that this low noise characteristic can be had with plastic resistors having a relatively low percentage of conductive particles in the resistor composition, thus retaining the desired feature of the resistor having the physical characteristics of the resin from which it is compounded. In the preferred forms of the present invention the resistor is molded from a mixture of a solvent soluble or dispersible thermosetting one step resin, such as a phenol aldehyde resin, as the non-conductive resin, and finely divided conductive powder material, such as carbon black.

A basic feature of the present invention resides in the manner in which the surface of the resistor wiper contact track, that is the area of the resistor surface that is traversed by the rheostat or potentiometer wiper, is formed. In general, the wiper contact track surface is formed from a viscous pasty mixture of the resin and conductive powder in a solvent for the resin, the resin being dissolved in the solvent and the powder being homogenously dispersed in the solution. The paste is deposited as a thin Patented Mar. 8, 1966 ice layer in the desired configuration of the rheostat or p0- tentiometer wiper contact track, and the solvent is evaporated. The residual film is then molded to a base, which may be composed either entirely of non-conductive plastic, or include an additional resin-conductive powder resistive current carrying portion. The film and base are comolded under curing heat and molding pressure, with a smooth mold surface applied along the wiper contact surface of said deposited layer. In order to prevent the base from punching through said deposited layer, the flow characteristics of the base and deposited layer resins should be of the same order of magnitude, and preferably substantially similar. The depositing and molding of the contact layer may be accomplished in two ways. A mold may be used having a recessed groove corresponding to the desired shape of wiper track. The resin-solvent-conductive powder layer may be deposited in the groove and there dried until the solvent is evaporated. A thermosetting molding resin powder, or molded preform, is then charged into the mold over the deposited film to provide a base for the resistor, and the entire charge heat cured and pressure molded to form an integrated resistance unit mounted on a base, whose exposed resistance surface is available as a low noise track for a conventional rheostat or potentiometer wiper contact. Alternatively, when the base member is formed as a preform, the resin-solvent-conductive powder mixture may be wiped over a desired surface area of the preform, and the thus applied layer permitted to dry until substantially all of the solvent has evaporated. The entire unit is then placed in a mold corresponding to the shape of the preform, and preferably the same mold as was used to form the preform, with the applied layer in contact with a smooth surface of the mold. This unit is there heat cured and molded under pressure to provide an integrated resistance track mounted on a suitable base. The resultant exposed surface of the resistance material provides a low noise track for a conventional rheostat or potentiometer wiper contact.

In either of the foregoing procedures, the base member may be composed entirely of non-conducting resin, so that the deposited surface layer constitutes the entire resistance element, or the base unit may include a conductive plastic portion to which the stated deposited layer is applied merely as a'surface element, to afford the desired low noise characteristic to traverse by a potentiometer or rheostat wiper contact.

The current carrying track of a resistor produced in accordance with the present invention is strong, tough and plastic-like in physical characteristics, displaying properties similar to the resin employed. Further, when used as a potentiometer or rheostat, it exhibits very low wiper contact noise, and retains its characteristics even after prolonged usage of over million cycles of operation. Such resistors have been produced with a wide range of resistivity characteristics, ranging from a volume resistivity of about 0.01 ohm-centimeter to about 20 ohm-centimeters.

It is accordingly one object of the present invention to provide a molded conductive plastic resistance unit, having the physical properties of plastic and a low surface noise characteristic whn traversed by a Wiper contact.

Another object of the present invention is to provide a surface layer for molded conductive plastic resistors,

having the physical properties of plastic and a low surface noise characteristic when traversed by a wiper contact.

Further objects of the present invention are to provide molded conductive plastic resistors which display low wiper contact surface noise characteristics, and hence are partiularly adapted to be used as potentiometers and rheostats, and to provide methods of producing these resistors.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following exemplary specific examples thereof had in conjunction with the accompanying drawings, in which like reference characters refer to like or corresponding parts, and wherein:

FIG. 1 is a vertical sectional and partial perspective view of a mold employed in fabricating one form of resistor in accordance with the present invention;

FIG. 2 is a cross-sectional and partial perspective view of a plastic preform fabricated from the mold of FIG. 1;

FIGS. 3 and 4 are cross-sectional and partial perspective views of two embodiments of plastic resistors fabricated in accordance with the present invention in the mold of FIG. 1; and

FIG. 5 is a molded resistor potentiometer fabricated in accordance with the present invention.

For the purposes of illustration, the form of the resistance unit described in this specification is chosen to be a raised annular track mounted on a circular base, so as to be adapted for use in a rotary type potentiometer or rheostat. Accordingly a representative type of mold for forming the unit is shown in FIG. 1. The mold comprises an annular sleeve housing a bottom or base member 11, defining a molding cavity 14 thereabove. Base member 11 has a recess 13 therein for defining the resistance track. A plunger 12 is axially slidably recieved in the cavity 14. When the mold cavity is appropriately charged with a plastic molding composition and the plunger 12 is positioned on top thereof, the charged mold is placed between a pair of heated pressure platens acting to compress the charge through the plunger 12 and heat cure the same, all as fully understood by those skilled in the art.

As previously stated, a basic principle of the present invention is to apply to the resistance track a surface layer which provides a low surface noise characteristic for a traversing potentiometer wiper contact. In accordance with one form of the invention, thissurface layer may be formed in the mold, but left in an uncured state. Whereupon the base member may be directly formed thereon by appropriately charging the mold on top of the deposited surface layer, and com olding and curing the joint charge. The following Examples I and II are specific examples of this procedure for forming the resistor, and of the resistor thus produced.

Example I 63 parts by weight of a phenol aldehyde one step thermosetting resin in lump form (Durez #175 resin) is dissolved in 75 parts by weight of a mixture of methyl ethyl ketone and cyclohexanone. To this solution is added 37 parts by weight of a conductive oil furnace carbon black (Vulcan XC-72R obtained from Godfrey L. Cabot, Inc), which has been previously dried by baking for 2 /2 hours at 150 C. and then cooled in a dessicator. The resultant mixture is mixed in a colloid mill until a lumpless homogenous viscous paste is obtained. A layer of this mixture of carbon black and resin dissolved in solvent is deposited in the recessed track 13 of mold base 11, which may be done with the aid of a spatula, and the layer is then heated below ouring temperature until substantially all of the solvent is evaporated. The dried layer now forms a film or deposit along the bottom portions of the recess 13. The base member 11 and sleeve 10 are then assembled as shown in FIG. 1, the mold cavity is charged to a desired depth with the Durez #175 molding powder, the plunger 12 is inserted, and the mold is placed in a press with platens at about 350 F. and exerting a molding pressure on the mold of about 500 lbs. p.s.i. for ten minutes, followed by 8000 lbs. p.s.i. for thirty minutes. The prodduct is then seasoned or stabilized by a further heating at at 75 C. without pressure for about twenty four hours.

The resultant resistor unit is shown in FIG. 3, and comprises a non-conductive polymerized phenol aldehyde base 15 having a raised non-conductive ridge 16. On the surface of the ridge 16 is the conductive layer 17 of polymerized phenol aldehyde containing carbon black. It should be noted that the layer 17 was formed by first being permitted to set with what was to become its wiper contact surface in contact with the mold recess 13, this recess of course having been machined smooth; and when pressure molded and cured, the contact surface was likewise in pressure contact with the surface of recess 13. Thus, the resistor is a layer which was originally formed from a viscous pasty solution of the resin in a solvent, the solution containing a desired proportion of conducting powder, and the resistor was finished by comolding under curing conditions with the base member, the surface of the resistor being formed by pressure contact with the corresponding mold surface.

The resistor produced by Example I is a strong plasticlike solid with good toughness, and physically is very similar to the polymerized phenol aldehyde when molded alone. When employed as a potentiometer with a conventional metal Wiper contact, the wiper surface contact noise is very low, and the resistor does not show any noticeable wear or change in characteristics even after million cycles of operation. The volume resistivity of the resistor produced by Example I is approximately 0.1 ohm-centimeter.

Example II A resistor can be prepared by following the identical procedure as set forth in Example I, except a medium flow channel carbon black (ELF0 obtained from Godfrey L. Cabot, Inc), is substituted for the Vulcan XC-72R carbon black, and the ratio of resin to carbon black to solvent is 70 parts by weight of resin, 30 parts by weight of carbon black and 75 parts by weight of solvent. The properties and structure of the resistor produced by this example are substantially the same as had with Example 1, except that the resultant unit has a volume resistivity of about 5 ohm-centimeters.

A variation of the procedure for forming the low noise surface layer on the resistance unit resides in first forming a preform of the unit with the ridge 16. The ridge 16 may include a conductive resistance section formed by prior art techniques, or not, as desired. The low noise layer is then applied to the top surface of the ridge, i.e. the wiper contact track, and the combined unit is then molded under pressure, and heat cured in the mold of FIG. 1. A specific example of this procedure for forming the resistor, and of the resistor thus produced, is as follows:

Example III A charge of 88 parts by weight Durez phenol aldehyde resin and 12 parts by weight of Vulcan XC-72R carbon black are intimately mixed in a dry state in a ball mill for 24 hours, the Vulcan XC-72R having been dried as set forth in Example I. Recess 13 is then filled with this mixture, and the mold cavity is further charged with a desired quantity of Durez #175 phenol aldehyde resin. This combined charge is then molded and partially cured under heat and pressure, the assembled mold being subjected to a press platen temperature of about 350 F. and pressure of about 500 lbs. p.s.i. The mold is subjected to this pressure and temperature for only fifteen minutes, to effect only partial curing, and hence provide a preform. The preform is then removed from the mold, and is shown in FIG. 2. In this instance, because of the conductive nature of the first portion of the resin charge, the rib 16 has a conductive portion 18. The conductive portion 18 formed in this manner, or even when fully cured. in the mold, has a very high wiper contact surface noise characteristic. In order to put a low noise wiper contact,

track on the unit, a further conductive plastic composition is compounded. 70 parts by weight of Durez #175 phenol aldehyde resin are dissolved in 75 parts by weight of a mixture of methyl ethyl ketone and cyclohexanone, and to this solution is added 30 parts by weight of ELF-0 carbon black. This mixture is intimately mixed in a colloid mill until a lumpless, homogeneous, viscous paste is obtained. The upper surface of rib 16 on the preform is coated with this latter mixture by daubing or wiping with a felt wiper. This coated preform is then dried in a hot air oven for 20 minutes at 90 C., and when the solvent has evaporated from the coating, the unit is returned to the mold, and the mold is again placed in the press. The preform charge is then subjected to molding and curing conditions of about 350 F. at 500 lbs. p.s.i. for ten minutes, followed by 8000 lbs. p.s.i. for thirty minutes, and the product is then seasoned or stabilized by a further heating at 75 C. without pressure for about twenty-four hours.

The resultant resistor had substantially the same physical properties as the unit produced by Example I, and a volume resistivity of about 20 ohm-centimeters. This resistor unit is shown in FIG. 4, and comprises the nonconductive base member 15, supporting an annular upstanding rib 16. The portion 18 of the rib is a resistive current carrying portion of high resistivity, and on the upper surface thereof is the low noise surface layer 17 of relatively low resistivity, the latter forming the wiper contact track for potentiometeror rheostat purposes. In this instance the wiper contact track surface is formed by applying a very thin surface layer of a conductive powder carrying resin solution to a preform, permitting this surface layer to set, and then pressure molding and curing the combined preform and surface layer with said surface bearing against a smoothly machined contour of the mold. In this manner, a surface is obtained for the overall high specific resistivity resistor having a very low contact noise ratio when traversed by a conventional potentiometer or rheostat wiper contact member.

It is apparent that in Example III the preform could be formed without the conductive section 18 on rib 16, and the entire current carrying path for the resistor could be obtained from the low noise coating 17 applied thereto. However, the coating applied by the method of Example III normally is a very thin coating with very low current carrying characteristics. Accordingly, for most applications it is desirable to provide the additional current carrying resistance section 18, as specifically described in Example III.

The teaching of Example IH can, however, be combined with that of Example I, to produce an adequate current carrying resistor without the use of the auxiliary current carrying layer 18. To this end, the preform of FIG. 2 is produced in accordance with the teachings of Example III, except the entire charge going to make up the preform is only non-conductive Durez #175 phenol aldehyde resin. Thus, the preform comprises the base member 15 and the upstanding rib 16, all composed of the same composition, i.e. the non-conducting resin entirely free of a conducting material. After the preform is set and removed from the mold, and homogenous pasty solution of the resin in a solvent, having a desired percentage of carbon black or other conducting powder admixed therewith, is laid in the recess 13 and heated to remove the solvent, as in Example I. Theprefonm is then returned to the mold, and under molding pressure and curing heat the preform and surface coating are comolded, whereby a resistor 17 with a low noise wiper contact surface is formed on the ridge 16.

By way of further explanation of the present invention, in order that those skilled in the art may more readily practise the invention in its various forms without restriction to the specific formulations provided in the examples, the following principles of the invention are set forth. Resins other than Durez 175 and other than this specific phenol formaldehyde resin may obviously be employed. The basic criteria in choosing a suitable resin are that it be a thermosetting one step resin, and that it possess a very long flow (of say greater than about 30 seconds and preferably of the order of -90 seconds), a medium cure, and a rigid set. The resin solvents should of course be volatile, and may desirably be alcohols, ketones, or mixtures thereof. Obviously, the solvent selected should not be so volatile as to hinder proper operation of the procedures by drying too fast, and the selection of an appropriate solvent will be apparent to those skilled in the art to suit the conditions under which the procedures are being performed. A mixture of methyl ethyl ketone and cyclohexanone as well as other alcohols and/or keton es are preferred. Acetone and methyl ethyl ketone alone are generally found to evaporate too rapidly, while isophorone is generally found to evaporate too slowly.

It is not intended to restrict the scope of the present invention to the specific examples and modifications herein expressly described, for other modifications of the processes and resistors described will be apparent to those skilled in the art. For example, other non-conducting thermosetting one-step resins than phenol aldehyde can obviously be employed. Similarly, other types of carbon black, and powdered conducting materials other than carbon can be employed. All as is well understood by those skilled in the art. The proportions of conductive powder to resin can be varied over wide ranges from those given in the specific examples, since the choice of proportions is governed primarily by the resi-stivity characteristics desired. It is considered, however, that the consistency of resin-solvent-conductive powder mixture plays a very important role in determining low noise contact surface characteristics of the resistor. Although this characteristic is certainly not limited to the specific proportions of the examples, the proportions should be such as to provide a very viscous and pastelikeconsistency to the solvent mixture. It is preferred in this respect that the amount of solvent used be not substantially in excess of that required to bring the resin into an homogenous paste-like solution or dispersion.

In large measure, the purpose for providing resistors with surfaces having low wiper contact noise characteristics, is for use in potentiometers and rheostats. Accordingly, FIG. 5 shows a resistance unit of the present invention embodied in a potentiometer. unit comprises the integrally molded insulating or nonconductive base 15 with the current carrying resistance 17 thereon. The ends of the resistor 17 are terminated in appropriate terminals 21 and 22.. An aperture in base 15 on the center of the resistor arc mounts a shaft 20 carrying wiper 19 having the contact element 23. Rotation of shaft 20 causes contact element 23- to traverse the low noise surface 17 of the resistor, and the contact 23 thus affords a variable tap for the potential applied across the resistor through terminals 21 and 22.

' Having thus described the present invention both generally and specifically, and in such terms and examples as would enable one skilled in the art to practice the same, it is intended that the scope of the invention shall be determined from the spirit and intent of the appended claims.

' What is claimed is:

1. A method of forming a molded resistor unit, comprising the steps of: depositing a layer, of a viscous solution of thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; converting said deposited solution to a sub stantially dried layer by evaporating the solvent therefrom; and comolding in a mold under pressure and curing said dried layer with a base member therefor comprised of a thermosetting resin material; said comolding step being elfected with a surface of said dried layer The resistance 7 in molding contact with a smooth molding surface of the mold.

2. A method as set forth in claim 1, wherein said powdered conductive material is carbon.

3. A method as set forth in claim 1, wherein said resin for said layer and said resin for said base member is a phenol aldehyde resin.

4. A method as set forth in claim 1, wherein said powdered conductive material is carbon, and wherein said resin for said layer and said resin for said base member is a phenol aldehyde resin.

5. A method of forming a molded resistor unit, comprising the steps of: depositing on a mold surface a layer, of a viscous solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; converting said deposited solution to a substantially dried layer by evaporating the solvent there from; charging the mold with a thermosetting resin material adapted to provide a base member for said dried layer; and comolding under pressure and curing said dried" layer with said base material.

6. A method as set forth in claim 5, wherein said base material is charged into said mold as a resin powder.

7. A method as set forth in claim 5, wherein said base material is charged into said mold as a preform.

8. A method of forming a molded resistor unit, comprising the steps of: forming a base member for said unit as a preform comprised of a thermosetting resin material; depositing on a surface portion of said preform a layer, of a viscous solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; converting said deposited solution to a substantially dried layer by evaporating the solvent therefrom; and comolding under pressure and curing said dried layer and preform in a mold with a surface of said dried layer in pressure contact with a smooth molding surface of the mold.

9. A molded resistance unit comprising: a base formed of molded thermosetting nonconducting material; and a molded current carrying resistor comprising a thermosetting material carried by said base; at least the surface of said resistor having been formed from a deposited layer of a viscous solution of a thermosetting resin in a solvent having admixed therewith a powdered conductive material; said layer having been first dried by evaporation of solvent; and then comolded in a mold under pressure and cured with said base, with the surface of said layer in molding contact with a smooth surface of said mold.

10. A potentiometer comprising: a molded resistance unit having a base formed of molded. thermosetting nonconducting material; and a molded current carrying resistor comprising a thermosetting material carried by said base; at least the surface of said resistor having been formed from a deposited layer of a viscous solution of a thermosetting resin in a solvent having admixed therewith a powdered conductive material; said layer having been first dried by evaporation of solvent and then comolded in a mold under pressure and cured with said base, with the surface of said resistor in molding contact with a smooth surface of said mold; and a wiper arm having a wiper contact member mounted to traverse the resistor; the surface of said resistor having a low surface noise characteristic when traversed by said. wiper contact.

ll. A potentiometer as set forth in claim 10, and further including a pair of terminals in electrical contact with spaced points of said resistor for applying a potential across said resistor.

12. In the manufacture of a resistor unit, the method of forming a contact wiper surface therefor comprising the steps of: depositing a layer of a viscous solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith; converting said deposited solution to a substantially dried layer by evaporating the solvent therefrom; and molding under pressure and curing said dried layer in a mold; said molding stop being effected. with a surface of said dried layer in molding contact with a smooth molding surface of the mold.

13. A method of forming a molded resistor unit, comprising the steps of: depositing a layer, of a fluid solution of a thermosetting resin in a solvent having a powdered. conductive material homogeneously admixed therewith, in a shape approximating that desired for the resistor; converting said deposited solution to a substantially dried layer by evaporating the solvent therefrom; and comolding in a mold under pressure and curing said dried layer with a base member therefor comprised of a thermosetting resin material; said comolding step being effected with a surface of said dried layer in molding contact with a smooth molding surface of the mold.

14. A molded resistance unit comprising: a base formed of molded thermosetting nonconducting material; and a molded current carrying resistor comprising a thermosetting material carried by said base; at least the surface of said resistor having been formed from a deposited layer of a fluid solution of a thermosetting resin in a solvent having admixed therewith a powdered conductive material; said layer having been first dried by evaporation of solvent; and then comolded in a mold under pressure and cured with said base, with the surface of said layer in molding contact with a smooth surface of said mold.

15. In the manufacture of a resistor unit,-the method of forming a contact wiper surface therefor comprising the steps of: depositing a layer of a fluid solution of a thermosetting resin in a solvent having a powdered conductive material homogeneously admixed therewith; converting said deposited solution to a substantially dried layer by evaporating the solvent therefrom; and molding under pressure and curing said dried layer in a mold; said molding step being effected with a surface of said dried layer in molding contact with a smooth molding surface of the mold.

References Cited by the Examiner UNITED STATES PATENTS Re. 18,818 5/1933 Rubin 338-217 1,889,379 11/1932 Rubin 338-217 1,992,529 2/1935 Henry 338-217 2,056,928 10/1935 Magdziarz 338-217 2,134,870 11/1938 Fruith 338-217 2,700,719 1/1955 Coler et a1 338-308 X 2,945,266 7/ 1960 Mainardi 18-59 2,999,991 9/1961 Bender 338-162 3,054,076 9/1962 Treff 338-162 3,064,314 11/1962 Cagne et al. 18-59 RICHARD M. WOOD, Primary Examiner. 

1. A METHOD OF FORMING A MOLDED RESISTOR UNIT, COMPRISING THE STEPS OF: DEPOSITING A LAYER, OF A VISCOUS SOLUTION OF THERMOSETTING RESIN IN A SOLVENT HAVING A POWDERED CONDUCTIVE MATERIAL HOMOGENEOUSLY ADMIXED THEREWITH, IN A SHAPE APPROXIMATING THAT DESIRED FOR THE RESISTOR; CONVERTING SAID DEPOSITED SOLUTION TO A SUBSTANTIALLY DRIED LAYER BY EVAPORATING THE SOLVENT THEREFROM; AND COMOLDING IN A MOLD UNDER PRESSURE AND CURING SAID DRIED LAYER WITH A BASE MEMBER THEREFOR COMPRISED OF A THERMOSETTING RESIN MATERIAL; SAID COMOLDING STEP BEING EFFECTED WITH A SURFACE OF SID DRIED LAYER IN MOLDING CONTACT WITH A SMOOTH MOLDING SURFACE OF THE MOLD. 